1. Field of the Invention
The present invention relates to a hydraulic braking system for use in an automotive vehicle, and more particularly to a hydraulic braking system which comprises a hydraulic booster for actuating a master cylinder in response to operation of a brake pedal with a hydraulic power pressure supplied from a power source.
2. Description of the Prior Art
In conventional service braking systems for an automotive vehicle, there are provided a plurality of hydraulic circuits connecting wheel brake cylinders mounted on road wheels with a hydraulic braking pressure generator such as a master cylinder, so that when one of the hydraulic circuits is broken, braking operation is performed by the rest of the hydraulic circuits. In general, a tandem master cylinder is used in a conventional dual circuits system.
In order to reduce a depressing force applied on a brake pedal in braking operation, the hydraulic braking system is provided with a servo unit which is referred as a servo or a booster and which utilizes compressed air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure (for a hydraulic booster) as a power source. The hydraulic booster is a booster which actuates the hydraulic braking pressure generator such as the master cylinder by the hydraulic power pressure supplied from the power source in response to depression of the brake pedal. In Japanese Patent Laid-open Publication No. 59-209948, for example, disclosed is a hydraulic braking system in which the hydraulic booster is associated with a tandem master cylinder and which operates as a conventional tandem master cylinder when the hydraulic booster does not operate.
Further, It has been proposed that a hydraulic pressure generated by the hydraulic booster in response to depression of the brake pedal is applied directly to one of the hydraulic circuits. For example, as shown in Japanese Patent Laid-open Publication No. 59-227552, the hydraulic pressure generated by the hydraulic booster is applied to rear wheel brake cylinders in a front-rear dual circuits system in order to reduce the stroke of the brake pedal.
As for the above-described conventional hydraulic braking system, in the case where the hydraulic booster becomes insufficient to fulfill its boost function, or the case where the hydraulic power pressure disappears due to stoppage of the power source or other defects so that the hydraulic booster becomes incapable of obtaining any boost function, the braking force on the front road wheels is ensured by the master cylinder, but a large depressing force shall be applied on the brake pedal in order to obtain the necessary braking force.
In the Japanese Patent Laid-open Publication No. 62-149547, there is disclosed a system, in which a booster or an auxiliary cylinder is arranged in parallel with a hydraulic braking pressure generator and a control valve is provided for applying to wheel brake cylinders a hydraulic braking pressure from a master cylinder increased by the auxiliary cylinder when a hydraulic pressure generated in a hydraulic pressure chamber of a hydraulic booster or a power source is less than that in the master cylinder by a predetermined value or above, while applying the hydraulic braking pressure from the master cylinder to the wheel brake cylinders without increasing the hydraulic braking pressure in the case other than the above. Thereby, even if the boost function of the hydraulic booster cannot be obtained, the hydraulic braking pressure from the master cylinder is increased by the auxiliary cylinder to be applied to the wheel brake cylinders, so that a large depressing force does not have to be applied on the brake pedal.
However, in the above described hydraulic braking system, there must be provided an auxiliary cylinder or the like in order to ensure a braking force when a source for reducing the stroke of the brake pedal, such as the hydraulic booster does not operate. As for the auxiliary cylinder, various embodiments thereof are disclosed in the above publication. However, any of those auxiliary cylinders must be incorporated into the conventional hydraulic braking system having the tandem master cylinder and the hydraulic booster for example, integrally or separately, so that the braking system as a whole tends to be expensive, large and heavy.